HP Smart Array 6i Controller and 128MB BBWC - Manual (Page 45)

Drive Arrays and Fault-Tolerance Methods 45
NOTE: When there are only two physical drives in the array, this fault-
tolerance method is often referred to as RAID 1.
Advantages:
•
Has the highest read and write performance of any fault-tolerant
configuration.
•
No data is lost when a drive fails, as long as no failed drive is mirrored to
another failed drive (up to half of the physical drives in the array can fail).
Disadvantages:
•
This method is expensive (many drives are needed for fault tolerance).
•
Only half of the total drive capacity is usable for data storage.
RAID 5—Distributed Data Guarding
In a RAID 5 configuration, data protection is provided by parity data (denoted
by Px,y in the figure). This parity data is calculated stripe by stripe from the user
data that is written to all other blocks within that stripe. The blocks of parity data
are distributed evenly over every physical drive within the logical drive.
When a physical drive fails, data that was on the failed drive can be calculated
from the remaining parity data and user data on the other drives in the array. This
recovered data is usually written to an online spare in a process called a rebuild.
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